It has become increasingly desirable to increase the efficiency and reduce the size of power-producing or thrust-producing devices such as gas turbine engines in aircraft. Gas turbine engines typically include one or more shafts that include compressors, bypass fans, and turbines. Typically, air is forced into the engine and passed into a compressor. The compressed air is passed to a combustor, and at high temperature and pressure the combustion products are passed into a turbine. The turbine provides power to the shaft, which in turn provides the power to the compressor and bypass fan or gearbox. Thrust is thereby produced from the air that passes from the bypass fan, as well as from the thrust expended in the turbine combustion products. This system is typically packaged together with power production and thrust generation co-located.
However, air can be thermodynamically inefficient, especially during high altitude operation of the engine (such as in an aircraft application). Air that enters the engine is of low pressure, therefore low density. In order to reach the needed pressure and temperature at the combustor exit, the air is compressed to very high pressure ratios and heated up to very high temperatures in the combustors. In order to provide adequate mass flow rate, significant volume flow rate of the low density air is pumped through high pressure ratio consuming significant amount of power. As a result the engines are made of large and heavy components, consume large amount to fuel, and may include significant operational and maintenance expenses to cope with high combustion temperatures.
To increase system efficiency and reduce component size and complexity of turbomachinery, some power-producing or thrust-producing use a closed cycle super-critical carbon dioxide (s-CO2) system. This system provides significantly improved efficiencies compared to Brayton and other air-based systems by operating in a super-critical region (operating at a temperature and pressure that exceed the critical point). That is, a phase-diagram of CO2, as is commonly known, includes a “triple point” as the point that defines the temperature and pressure where solid, liquid, and vapor meet. The critical point is the top of the dome made up of the saturated liquid and saturated vapor lines. Above the critical point is the gaseous region. At the triple point the fluid can exist in liquid, vapor, or in a mixture of the both states. However, at higher temperature and pressure, a critical point is reached which defines a temperature and pressure where gas, liquid, and a super-critical region occur.
Fluids have a triple point, a critical point, saturated liquid and vapor lines, and a super-critical region. One in particular, carbon dioxide, is particularly attractive for such operation due to its critical temperature and pressure of approximately 31° C. and 73 atmospheres, respectively, as well as due to its lack of toxicity. Thus, s-CO2—based systems may be operated having very dense super-critical properties, such as approximately 460 kg/m3. The excellent combination of the thermodynamic properties of carbon dioxide may result in improved overall thermodynamic efficiency and therefore a tremendously reduced system size. Due to the compact nature and high power density of a power source that is powered with a super-critical cycle, the overall size of the engine may be significantly reduced, as well.
Aircraft typically include auxiliary loads that are powered by electrical, hydraulic, and pneumatic sub-systems that provide power to mechanical loads, actuators, and the like. The electrical sub-systems may be powered by electrical generators, which are thermodynamically inefficient because of the conversion from heat (typically of the gas turbine engine), to electrical power, and then provided to the auxiliary loads. Further inefficiencies may result from storage of the electrical energy as chemical energy as in a battery, as an example. In addition, in an aircraft application additional overall system inefficiencies occur because of the mass of equipment that is typically used (electrical generator, batteries, etc.) to convert and store the energy for auxiliary operation. Similar conversion, distribution, and storage inefficiencies are present for hydraulic and pneumatic distribution systems as well.
As such, it is desirable to reduce overall mass and improve system efficiency when employing a s-CO2 system.